Mixed phase pigments with a rutile structure have long been known. Mixed oxides with a rutile structure can be produced in color shades extending over wide ranges of the visible spectrum by incorporating metal oxides such as NiO, Cr.sub.2 O.sub.3, CuO and MnO with Sb.sub.2 O.sub.5, Nb.sub.2 O.sub.5 and WO.sub.3 in the crystal lattice of the titanium dioxide.
If the color producing metal ion which has been incorporated has a valency other than 4 then another metal oxide of a different valency is also incorporated in the rutile lattice to equalize the valency so that the incorporated metal oxides in the lattice satisfy the following condition: ##EQU1## It follows from this that for a and b: EQU a.n+b.m=4(a+b)
In the above equations:
A: denotes an incorporable metal such as Cr.sup.3+, Ni.sup.2+, Cu.sup.2+ or Mn.sup.3+, PA1 B: denotes an incorporable metal such as Sb.sup.5+, Nb.sup.5+ or W.sup.6+, PA1 O: stands for an oxygen atom, PA1 n: denotes the valency of the incorporated metal A, where n=2 or 3, PA1 m: denotes the valency of the incorporated metal B, where m=5 or 6 and PA1 a and b stand for small whole numbers.
These ratios are generally for the most part observed but rutile mixed phase pigments which deviate considerably from this idealized composition are also known.
Nickel- and chromium-rutile mixed phase pigments have achieved considerable technical importance. When nickel oxide and chromium oxide are incorporated in TiO.sub.2 as color producing oxides, metal oxides of a higher valency are also incorporated for valency equalization, especially oxides of antimony but also oxides of niobium and/or tungsten (Nos. US-A-1 945 809, US-A-2 251 829, US-A-2 257 278, US-A-3 832 205).
Pigments of this type are produced from anatase and/or hydrates of titanium dioxide with incorporable metal oxides or compounds giving rise to these metal oxides by annealing them at temperatures of about 900.degree. to about 1200.degree. C. and then grinding the annealed products (US-A-3 022 186, Ullmanns Encyklopadie der technischen Chemie, 4th Edition, Volume 18 (1979), pages 608 to 609).
The rutile mixed phase pigments have a high resistance to light, weathering, acids and alkalis as well as being resistant to other chemicals and they are also stable at temperatures of up to about 1000.degree. C. Further details may be found in "Lehrbuch der Lacke and Beschichtungen, Volume II, Pigmente, Fullstoffe, Farbstoffe", by Dr. Hans Kittel, published by W. A. Colomb in the publishing company W. Heinemann GmbH in 1974, Berlin Oberschwandorf, pages 92 to 95.
The excellent stability of the color tone on exposure to light and weathering even in very light shades, for example when lacquers based on alkyl-melamine resins or silicone polyesters are exposed to weathering, predestine these pigments for the pigmentation of stoving lacquers or coil coat lacquers.
The pigmentation of plastics with rutile mixed phase pigments is also gaining increasingly in importance.
The rutile mixed phase pigments which have hitherto been of the greatest economical importance are those which contain chromium (III) oxide and in which antimony oxide in particular but also nioium and/or tungsten oxide are incorporated as metal oxides of a higher valency.
The object of various patent applications for such rutile mixed phase pigments containing chromium (III) oxide is a process of preparation providing pigments with improved coloristic properties such as color purity, saturation, intensity and covering power. Thus rutile nuclei which have been prepared separately before calcining of the mixture of raw materials are capable of increasing the purity of the color (DE-A-2 605 651). In USSR Patent Application SU-A 1 198 085, rutile mixed phase pigments containing chromium and antimony are improved in their covering power and color intensity by adding 2 to 10% of non-coloring chlorides of the first and second group of the periodic system before the mixture of raw materials is calcined.
The known mineralizers for rutile mixed phase pigments, such as lithium fluoride or sodium fluoride, for example, which are mentioned as of equal value in US-A-3 022 186, are known to be used for facilitating the reactions of the solid substances by the presence of a fluid phase. Thus the presence of a mineralizer may reduce the time required for a solid body reaction and lower the temperature required or enable a more complete reaction to take place. The addition of alkali metal and alkaline earth metal chlorides according to SU-A 1 198 085 must be taken to serve the same purpose.
Chromium-antimony-rutile mixed phase pigments have the undesirable characteristic of being sensitive to chilling, i.e. to rapid cooling, which may result in a loss of color purity. This sensitivity to chilling is particularly pronounced in the darker, fuller types of pigments for which a larger quantity of color producing chromium oxide is added and/or higher calcining temperatures are employed. Lighter color pigments, which are generally prepared at somewhat lower temperatures, are less sensitive to chilling but still sensitive to such an extent that a difference in quality is noted compared with products which have been cooled more slowly.
Investigation of other rutile mixed phase pigments has surprisingly shown that these other pigments, e.g. nickel-antimony-rutile or chromium-tungsten-rutile mixed phase pigments were insensitive to rapid cooling.
The sensitivity to chilling described above is also found to occur when chromium-antimony-rutile mixed phase pigments are used for coloring plastics, especially those which require relatively high processing temperatures, such as polyamides or ABS plastics. When such pigments are heated to temperatures higher than about 260.degree. C. in the course of processing, they undergo undesirable color changes.
It is an object of the present invention to provide chromium-antimony-rutile mixed phase pigments whose colouristic properties are not impaired by rapid cooling, i.e. are not sensitive to chilling.